Sole-water heat pump with buffer tank and underfloor heating

I have meanwhile received a construction description for the offer from the local general contractor and I am not quite sure what to make of the heating system:

Execution with a geothermal heat pump. Installation of a Viessmann heat pump or similar.
Power according to heating load calculation. Deep drilling according to calculation by a specialist office.
Supply and installation of a 390-liter hot water storage tank and a 400-liter buffer tank.

Execution with thermal insulation according to heating system regulations. The heating pipes will be laid in the screed insulation, visible in the boiler room.
In the living rooms on the ground floor and upper floor, a underfloor heating system with individual room controls will be installed.

At first, I find the hot water storage tank quite large for 3 people, but okay, it can certainly be dimensioned smaller if that results in financially relevant differences. The bigger question mark I see is the buffer tank and whether it is needed at all. I did a bit of research but the articles I find on the internet usually relate to air-water heat pumps. Overall, it is considered rather unnecessary with underfloor heating, and unlike air-water heat pumps, geothermal water-water heat pumps do not have the problem of becoming less efficient once the sun goes down in winter. Is there any good reason to install a buffer tank with geothermal heating?

Is there any good reason to install a buffer tank with geothermal heating?

Yes, exactly one. Profit maximization of the heating engineer. A 390 l hot water tank for 3 people is clearly too large.

Buffer tank: unnecessary
390l hot water tank: depends. Simple tank with small heat exchanger - ok. As high-performance tank - unnecessary

The profit maximization of the contracted heating installer is definitely not a good reason for me ;) I will add that to the list for the next conversation

Hello,

we also have a brine heat pump, but without a buffer tank. It is not necessary.
The volume of the tank seems too large to me for 3 people as well, we have 300l and are five.
I would not want expressions like "or equivalent" in my construction specification, the exact product that will be installed in the house should be named. You can ask which alternatives there are to Viessmann and then choose the heating system you like best.